1. Field of the Invention
The present invention relates generally to increasing the accuracy of MWD nuclear based measurement systems. Specifically, the invention provides a secondary measurement system for determining the tool displacement from the bore-hole wall for calculated compensation of nuclear measurement data.
2. Prior Art
The measurement of formation properties during well drilling by "Measuring While Drilling" (MWD), systems provides the greatest accuracy and efficiency. Nuclear measurements such as neutron porosity and gamma-gamma density which employ a nuclear source and various spaced detector systems in an MWD tool, provide formation data which is fundamental for characterizing formation properties. Nuclear measurements, however, are limited in their accuracy in MWD systems by displacement from the bore-hole wall. The variability of displacement of the tool from the wall during drilling operations, combined with the influence of the drilling fluids present in the bore-hole on the detected values, limits usefulness of nuclear based measurement devices.
In wire line logging applications, displacement devices which force the measurement tool against the wall of the bore-hole are employed for increased accuracy. Disclosures demonstrative of this type of system are present in U.S. Pat. No. 3,023,507 to W. A. Camp, and U.S. Pat. No. 4,047,027 to Richard M. Bateman and Christian M. Clavier. Forced displacement is unsatisfactory for MWD application, and full gage stabilization of MWD tools has been employed to increase accuracy of nuclear base measurement systems. Full gage stabilization systems have included arrangements whereby the nuclear detectors are mounted in the fins of the stabilizers, or through the use of materials which are transmission transparent in the stabilizers which permit nuclear radiation to penetrate to sensors in the body of the tool. A system employing this arrangement is disclosed in U.S. Pat. No. 4,879,463 to Wraight et al. Full gage stabilization is impractical in some drilling applications rendering these techniques ineffective. Furthermore, when the bore-hole itself is out of gage even detectors on a stabilizer blade are inadequate as disclosed in the paper by D. Best, P. Wraight, and J. Holenka entitled "An Innovative Approach to Correct Density Measurements While Drilling For Hole Size Effect", Society of Professional Logging Engineers, 31 st Annual Logging Symposium, Jun. 24-27, 1990, paper G. Unconstrained motion of an MWD tool employing nuclear based measurement systems within the bore-hole results in a measurement response which is, at best, an average with varying amounts of drilling fluid present between the formation and the sensors for each measurement. This results in measurements with poor sensitivity to the properties of the formation, or requires the use of stronger sources which is undesirable from a cost and handling standpoint.
Nuclear measurement employs the statistical analysis of nuclear response or counts measured by the detector. Since data taken with the tool positioned closer to the bore-hole wall will represent the properties of the formation differently than measurements taken at greater displacement, and consequently, higher quantities of intervening drilling fluid, knowledge of the distance of the tool from the bore-hole wall may be employed to compensate the data for relative accuracy. An MWD detector response is not usually axisymmetric and the distance from the detector to the formation is required. It is therefore desireable to employ a secondary measurement device for determining the tool displacement from the bore-hole wall during nuclear measurement. The determination of the stand-off distance must be rapid with respect to the rotational speed of the MWD tool to allow accurate and timely measurement of the stand-off distance from the bore-hole wall when the tool is eccentered in the bore-hole. It is also desireable to simplify the compensation calculations by real time integration of nuclear data with stand-off distance measurement for efficiency in compensation calculation.